The present invention relates in general to the field of target flowmeters, and in particular to those having a uniform response over the transition between turbulent and laminar flow conditions.
Target flowmeters have long been used for the measurement of the rate of flow of liquids through a pipeline. They operate on the principle that the amount of force generated by onrushing fluid impacting on a target whose planar surface is oriented perpendicular to the direction of flow is proportional to the square of the flow velocity. The target typically is mounted on a pivotable balance beam to permit its deflection. However a force balance mechanism counteracts the movement of the balance beam and generates an output signal corresponding to the amount of force. This output signal in turn is correlated to a flow velocity measurement. Typical of such a conventional target flowmeter is the Model 13T Target Flow Transmitter manufactured by The Foxboro Company, Foxboro, Mass.
Although the target flowmeter has shown to be capable of acceptable accuracy in the region of turbulent fluid flow (i.e., pipe Reynolds numbers R.sub.D greater than 4,000) a noticeable dropoff in the accuracy occurs once the fluid flow has moved down into the laminar-to-turbulent flow transition region (i.e., pipe Reynolds numbers in the range 2,000-4,000). In the past, if a target flowmeter was to be used over both these ranges of fluid flow conditions, then some sort of compensation was required to insure accurate overall performance. Often "disturbing plates" were used to flatten out the laminar flow profile prior to impact with the target, to approximate the turbulent flow profile. However, such an approach produces an undesirable permanent decrease in the flow coefficient.
Also, the compensation schemes, whether mechanical or electronic in nature, often unduly complicated the flowmeter structure, thereby increasing its cost while adversely affecting its reliability and efficiency.
With this in mind, it is an object of the present invention to provide an improved target flowmeter by employing a target body having a more uniform response to flow regardless of the flow regime in which it is operating. It is a further object of the present invention to accomplish the above with a minimum of modification to an existing target flowmeter, to permit retrofitting thereof.
It is yet a further object of the present invention to achieve a target flowmeter having the above characteristics which is usable in a variety of fluids, including liquids, multi-component fluids, and non-Newtonian fluids.